Well, technically it isn't a word, but an acronym for 'pussy bitch'. It may pertain to someone who is unwilling to something, or acts in a way that is unappealing. PB can be used in anyway to describe anything, mainly people though. People who are PB's lack in courage, are scared to do new things or scared of people, or just generally suck. No one likes a PB, they're lame and...PB-like. When calling someone a PB it generally encourages them to do something they initially don't want to do :D

Are you crying?... Don't be a little PB!

DID KANYE WEST JUST INTERRUPT TAYLOR SWIFT?! THAT WAS SUCH A PB MOVE!

Ashley, WHY WON'T YOU TELL ME WHAT THEY SAID?! STOP BEING A PB! I WILL SUPER POKE YOU!